Electric utilities need to match generation to load, or supply to demand. Traditionally, this is done on the supply side using Automation Generation Control (AGC). As loads are added to an electricity grid and demand rises, utilities increase output of existing generators to solve increases in demand. To solve the issue of continuing long-term demand, utilities typically invest in additional generators and plants to match rising demand. As load levels fall, generator output to a certain extent may be reduced or taken off line to match falling demand. As the overall demand for electricity grows, the cost to add power plants and generation equipment that serve only to fill peak demand becomes extremely costly.
In response to the high cost of peaking plants, electric utility companies have developed solutions and incentives aimed at reducing both commercial and residential demand for electricity. In the case of office buildings, factories and other commercial buildings having relatively large-scale individual loads, utilities incentivize owners with differential electricity rates to install locally-controlled load-management systems that reduce on-site demand. Reduction of any individual large scale loads by such a load-management systems may significantly impact overall demand on its connected grid.
In the case of individual residences having relatively small-scale electrical loads, utilities incentivize some consumers to allow installation of demand-response technology at the residence to control high-usage appliances such as air-conditioning (AC) compressors, water heaters, pool heaters, and so on. Such technology aids the utilities in easing demand during sustained periods of peak usage.
Demand-response technology used to manage thermostatically-controlled loads such as AC compressors typically consists of a demand-response thermostat or a load-control switch (LCS) device. A demand-response thermostat generally controls operation of a load by manipulating space temperature. An LCS device can be wired into the power supply line of the AC compressor or other electrical load, and thereby interrupt power to the load when the load is to be controlled.
However, while the demand-response schemes described above shed demand during peak times, especially for systems utilizing AC units, that demand is often time-delayed and merely pushed to another time along the utility demand timeline. In other words, demand-response schemes are suitable for reducing peak loads, but do not always affect an actual decrease in energy usage. A key problem lies in the energy consumed by AC units typically used in thermostatically-controlled HVAC systems. A majority of the energy consumed by such a system is spent powering the AC compressor. In a recent Environmental Protection Agency report, it was reported that air conditioning accounts for 13% of total home energy expenses on average, and over 20% in hot, humid regions. This statistic is made more significant by the fact that AC units are typically used between three to five months per year, so their effect on the peak demand during summer periods is very significant.
An oversized AC unit exacerbates the problem of high-energy consumption by HVAC systems. The accurate sizing of HVAC equipment, and specifically, the AC unit, is often quite challenging. Many factors contribute to the proper sizing of an AC unit, including the angle at which the sun contacts the home, the type of windows installed in the home, the interior window shading of the windows, the insulation installed in the home, the air circulation patterns, the efficiency of the duct system, and the size of the living space, among others. In addition, those factors change over time as the home and landscaping ages. Because those involved with home construction or AC unit selection, like homeowners and homebuilders, do not want to undersize an AC unit and have to replace the unit later, AC units tend to be oversized. Additionally, oversized units typically provide cooling more quickly, thus avoiding any chance of not meeting the cooling demand of the occupants.
However, the oversizing of AC units contributes to the problem of energy overusage, among other issues. One problem is the short run times of oversized units where the units run for shorter periods of time than are engineered for optimum operation. The efficiency of air conditioners is low when first starting, and increases gradually, reaching peak efficiency in about 10 minutes for most residential AC units. (e.g. long enough for the unit to be running at optimum efficiency). In addition even a properly sized unit will have short run times on days where cooling demand is low.
A number of other problems arise because of short run times. Relatively short operation times followed by relatively long off times do not allow the HVAC system to effectively remove humidity. Improperly dehumidified air adversely effects home comfort, reduces AC cooling efficiency, and can also promote the growth of mold and mildew indoors. Likewise, short run times decrease overall air circulation, resulting in repercussions on air quality and home comfort. Perhaps most importantly, short run times costs homeowners and commercial building owners additional money to operate, as the units are not operating at peak efficiency and reduction in overall life of the unit because the number of AC unit cycles is directly related to a units life (more than just runtime hours).
One attempt at improving the energy-efficiency characteristics of HVAC systems relies on variable speed AC unit compressors and fans that may be used to increase system turndown. However, such technology remains relatively expensive for new HVAC units. Further, retrofitting existing, working HVAC units to replace “single speed” technology with variable speed technology does not provide a convenient nor cost-effective solution for improving energy efficiency.
Another attempt at improving AC system efficiency is described in U.S. Pat. No. 5,960,639 to Hammer, entitled “Apparatus for Regulating Compressor Cycles to Improve Air Conditioning/Refrigeration Unit Efficiency”. Hammer discloses methods and systems for addressing compressor short-cycling. Short-cycling occurs when the time between a compressor stopping then restarting is so short that coolant pressures within the HVAC system do not have time to equalize, and the compressor does not have time to cool. Such conditions may occur in undersized HVAC systems, and result in decreased system efficiency. While the invention disclosed by Hammer addresses efficiencies for systems experiencing short-cycling, often in undersized units, or on peak usage days, Hammer fails to address the energy inefficiencies caused by short run times (as opposed to short off times) occurring in oversized AC systems.
Thus, there remains a need for technology capable of reducing energy imposing efficiencies of existing, oversized HVAC systems.